Comparator circuit

ABSTRACT

A charge pump system includes a charge pumping circuit for outputting a high voltage V PP  at a node. An oscillator circuit, coupled to the charge pumping circuit, drives the charge pumping circuit with at least one clock signal. A current source generates a pulldown current. A voltage divider circuit is coupled between the node and the current source. The voltage divider circuit cooperates with the current source to form a feedback loop for controlling the oscillator circuit to run at variable, optimum frequency for controlling the rate-of-rise and the amplitude of the high voltage V PP  while minimizing power-supply current drain.

CROSS-REFERENCE TO RELATED PATENTS

1. This application relates to the subject matter disclosed in the following United States Patents:

2. U.S. Pat. No. 5,583,425, entitled “Voltage Comparator With Controlled Output Current Proportional To Difference Voltage;” and

3. U.S. Pat. No. 5,686,823, entitled “Bandgap Voltage Reference Circuit.”

4. The above patents are assigned to the present Assignee and are incorporated herein by reference.

TECHNICAL FIELD OF THE INVENTION

5. This invention relates generally to the field of charge pumps, and more particularly, to a low-current charge pump system.

BACKGROUND OF THE INVENTION

6. Many integrated circuit (IC) devices, such as nonvolatile memories, include a high-voltage generator for generating a voltage (V_(PP)) having a value greater than the supply voltage (V_(DD)). According to previously developed techniques, such a high voltage generator typically employs a charge pump driven by a free-running oscillator in conjunction with a voltage regulator. The oscillator may be implemented as a closed ring of an odd-number (e.g., seven or nine) of inverter stages. The voltage regulator limits the maximum voltage value for V_(PP) by comparing it—for example, via a resistance voltage divider circuit—to a fixed reference voltage. When the divider voltage exceeds the reference voltage, a shunt-I_(PP) current path is established to pull down V_(PP) until the comparator balance of the voltage regulator is satisfied.

7. Such previously developed techniques utilizing current-shunting to regulate V_(PP) are generally wasteful and inefficient. For example, although it is often desirable to have a rise time for V_(PP) in the range of tens to hundreds of microseconds, the current shunting technique relies on shunting away pump current in excess of that required to charge load capacitance at the desired rate. In addition, the oscillator used to drive the charge pump expends large amounts of voltage supply current I_(DD) (typically in the range of 2-5 mA), converting it to pump-output current I_(PP), usually at very low efficiency (I_(PP)/I_(DD)). Furthermore, because the time delay of the inverter ring of a typical oscillator is designed to produce the minimum frequency necessary for the charge pump to reach the desired V_(PP) under all possible conditions for voltage, temperature, and process, this minimum frequency is generally set to a value sufficiently high (e.g., in the range of 2-15 MHz) to ensure adequate control by the shunt regulator. For many sets of conditions, however, this minimum frequency produces current greatly in excess of functional circuit requirements, drawing higher supply current than is necessary.

SUMMARY

8. The disadvantages and problems associated with previously developed techniques for generating and regulating the output of a high voltage generator have been substantially reduced or eliminated using the present invention.

9. According to one embodiment of the present invention, a charge pump system includes a charge pumping circuit for outputting a high voltage V_(PP) at a node. An oscillator circuit, coupled to the charge pumping circuit, drives the charge pumping circuit with at least one clock signal. A current source generates a pulldown current. A capacitor voltage divider circuit is coupled between the node and the current source. The voltage divider circuit cooperates with the current source to form a feedback loop for controlling the oscillator circuit to run at variable, optimum frequency for controlling the rate-of-rise and the amplitude of the high voltage V_(PP).

10. According to another embodiment of the present invention, an oscillator circuit includes a current mirror which receives a control signal and generates an output current signal in response. An inverter is coupled to the current mirror. A capacitor receives the output current signal of the current mirror through the inverter. A NOR gate receives the voltage developed across the capacitor and generates a clock signal.

11. According to yet another embodiment of the present invention, a voltage divider circuit includes a first capacitor coupled to a first node at which a high voltage V_(PP) appears. A second capacitor is coupled in series to the first capacitor at a second node, and a third capacitor is coupled in series to the second capacitor at a third node. A source follower, coupled to the second node, translates a voltage at the second node to a voltage having a value of approximately one-half V_(PP).

12. According to still yet another embodiment of the present invention, a comparator circuit includes a first source follower which follows a supply voltage V_(DD). A second source follower follows a high voltage V_(PP). A current mirror is coupled to the first and second source followers. The current mirror pulls the voltage at a node to a first value if the high voltage V_(PP) is approximately equal to the supply voltage V_(DD), and pulls the voltage at the node to a second value if the high voltage V_(PP) is greater than the supply voltage V_(DD), thereby comparing the high voltage V_(PP) to the supply voltage V_(DD) without drawing current from the high voltage V_(PP).

13. A technical advantage of the present invention includes providing a charge pump system which outputs a V_(PP) with high voltage value and yet requires a minimal amount of supply current (I_(DD)) (e.g., less than 50 uA).

14. Other aspects and advantages of the present invention will become apparent from the following descriptions and accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

15. For a more complete understanding of the present invention and for further features and advantages, reference is now made to the following description taken in conjunction with the accompanying drawings, in which:

16.FIG. 1 is a schematic diagram in partial block form of a low-current charge pump system, in accordance with an embodiment of the present invention;

17.FIG. 2 is a schematic diagram of an exemplary embodiment for a charge pumping circuit;

18.FIG. 3 is a schematic diagram of an exemplary embodiment for an oscillator circuit, in accordance with an embodiment of the present invention;

19.FIG. 4 is a schematic diagram of a voltage divider circuit, in accordance with an embodiment of the present invention; and

20.FIG. 5 is a schematic diagram of an exemplary embodiment for a switch circuit and a comparator circuit, in accordance with an embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

21. The preferred embodiments of the present invention and their advantages are best understood by referring to FIGS. 1 through 5 of the drawings. Like numerals are used for like and corresponding parts of the various drawings.

22. With the present invention, a charge pump system outputs a V_(PP) with high voltage value and yet requires a total supply current (I_(DD)) of less than 50 uA. Approximately one-half of this current may be used for a reference voltage generator circuit (described below). Hence, the average current used for the oscillator circuit is less than 25 uA, which is substantially less (e.g., by a factor of 100) than with previously developed techniques.

23. To achieve this large reduction in current, the present invention utilizes one or more of the following strategies: (1) minimizing the number of moving nodes in the charge pump system, (2) minimizing the rate at which the voltage potential of the moving nodes changes, and (3) having no static current drain on the V_(PP) which is generated.

24. The first and second strategies minimize dynamic supply-current drain. In general, dynamic supply current (I_(dynamic)) is proportional to the total capacitance (C) being charged/discharged in the charge pump system, the charging voltage (V) which is typically supply voltage V_(DD), and the frequency (f) at which the current changes. In particular, the value for dynamic supply-current is given by the following equation:

I_(dynamic)=CVf.

25. To implement the first strategy, in one embodiment of the present invention, a charge pump system incorporates an oscillator circuit which is designed to minimize the number of moving nodes in the charge pump system. The oscillator circuit is generally the main source of V_(DD) dynamic-current drain in a charge pump system.

26. To implement the second strategy, in another embodiment of the present invention, a charge pump system incorporates a feedback loop. The feedback loop is used to control the frequency of a clock signal fed into a charge pumping circuit which outputs voltage V_(PP). The frequency of the clock signal is optimized during risetime of V_(PP) and goes to zero once a desired voltage level for V_(PP) is reached. The feedback loop comprises a number of different elements, the parameters for which are selected to produce the optimal frequency.

27. To implement the third strategy, in yet another embodiment of the present invention, a charge pump system incorporates a voltage divider circuit having a source follower powered by V_(PP). The source follower outputs a voltage having a value of approximately one-half V_(PP) on a corresponding bus. The half-V_(PP) bus is arranged to be pulled to ground when the charge pump system is not active. Accordingly, no pulldown is required for the source follower, and hence this circuit draws no current from V_(PP). Thus, a voltage of one-half V_(PP) is made available with no current drain on V_(PP).

28. Each of these embodiments for implementing the three strategies are described below in more detail.

29. Charge Pump System

30.FIG. 1 is a schematic diagram in partial block form of a low-current charge pump system 2, in accordance with an embodiment of the present invention. Charge pump system 2 can be incorporated into an IC device, such as an electrically erasable programmable read-only memory (EEPROM). Charge pump system 2 generally functions to generate a voltage V_(PP) that is higher than a supply voltage V_(DD). If charge pump system is incorporated into an EEPROM, voltage V_(DD) is used during normal operation, and voltage V_(PP) can be used during actual programming.

31. Voltage V_(PP) appears at a node A and may have a value of approximately 16V. Voltages V_(PP) and V_(DD) can be alternatively placed on a voltage V_(DD)/V_(PP) bus 4. In particular, a switch 6 is operable to connect voltage V_(DD) to V_(DD)/V_(PP) bus 4, and a switch 8 is operable to connect voltage V_(PP) to V_(DD)/V_(PP) bus 4.

32. Charge pump system 2 includes a charge pumping circuit 12 which outputs voltage V_(PP). Charge pumping circuit 12 may comprise a series of rectifying devices and, in one embodiment, can be implemented with capacitors and diode-connected transistors according to a conventional technique.

33. An oscillator circuit 14 is coupled to charge pumping circuit 12. As used herein, the terms “connected,” “coupled,” or any variant thereof, means any connection or coupling, either direct or indirect, between two or more elements. Oscillator circuit 14 generally functions to drive charge pumping circuit 12. In particular, oscillator circuit 14 may generate two non-overlapping, oscillating clock signals Φ₁ and Φ₂, each of which has a corresponding frequency and may comprise a square wave signal. Clock signals Φ₁ and Φ₂ are input into charge pumping circuit 12 to alternately apply voltages across the diode-connected transistors contained therein, thus pumping charge through the rectifying elements; this causes the voltage at node A to rise to the V_(PP) voltage level. The rate at which V_(PP) can be generated is directly proportional to the frequency of clock signals Φ₁ and Φ₂.

34. A comparator circuit 16 is coupled to oscillator circuit 14. Comparator circuit 16 functions to compare a scaled-down version of the V_(PP) voltage against a reference voltage (V_(ref)) described below. These voltages may be applied at nodes B and C, respectively, which are connected to the input terminals of comparator circuit 16. Comparator circuit 16 generates a control current I_(CNTL) signal, the value of which is proportional in magnitude to the difference between the reference voltage and the scaled-down V_(PP) voltage and which decreases to zero when the two voltages are equal. Control current I_(CNTL) signal is input into oscillator circuit 14 and establishes the frequency of clock signals Φ₁ and Φ₂ output by oscillator circuit 14. In one embodiment, comparator circuit 16 can be implemented as the circuit shown and described in U.S. Pat. No. 5,583,425, entitled “Voltage Comparator With Controlled Output Current Proportional To Difference Voltage,” which is incorporated herein by reference.

35. A reference voltage generator circuit 18 is coupled to one of the input terminals of comparator circuit 16 at node C. Reference voltage generator circuit 18 generates the reference voltage (V_(ref)) against which the scaled-down version of V_(PP) voltage is compared. The value of the reference voltage is preferably constant. In one embodiment, reference voltage generator circuit 18 can be implemented as the bandgap reference voltage generator shown and described in U.S. Pat. No. 5,686,823, entitled “Bandgap Voltage Reference Circuit,” which is incorporated herein by reference. Alternatively, reference voltage generator circuit 18 can be implemented according to conventional techniques well-known to those skilled in the art.

36. A voltage divider circuit 20 divides the voltage at node A. One result of such division is the scaled-down V_(PP) voltage. Voltage divider circuit 20 comprises a first capacitor 22 and a second capacitor 24 which have respective values of “C1” and “C2.” Capacitors 22 and 24 are connected in series between node A and a node D. A top terminal of voltage divider circuit 20 is coupled to the output terminal of charge pumping circuit 12 at node A. A tap-point terminal of voltage divider circuit 20 lies between capacitors 22 and 24. The scaled-down V_(PP) voltage appears at the tap-point terminal, and its voltage value is related to the values C1 and C2 of capacitors 22 and 24, respectively. A bottom terminal of voltage divider circuit 20 is located at node D.

37. A switch 26 switchably connects the tap-point terminal of voltage divider circuit 20 between nodes B and C, thereby alternately coupling the tap-point terminal to one input terminal of comparator circuit 16 and the output terminal of reference voltage generator circuit 18 (which coincides with the other input terminal of comparator circuit 16). A switch 28 switchably connects the bottom terminal of voltage divider circuit 20, which coincides with node D, between node C and a node E. This alternately couples the bottom terminal of voltage divider circuit 20 to the output terminal of reference voltage generator circuit 18 (one input terminal of comparator circuit 16) and a current source 30 (described below).

38. Current source 30 has an input terminal at node E which is switchably connected to voltage divider circuit 20. Current source 30 generates a pulldown current I_(pulldown) which functions to pull down the bottom terminal of voltage divider circuit 20.

39. Collectively, current source 30 and voltage divider circuit 20 support a feed-back loop for charge pump system 2. The values C1 and C2 of capacitors 22 and 24, respectively, in voltage divider circuit 20 can be selected so that the feed-back loop causes V_(PP) to rise at the desired rate and to be limited to the desired amplitude by nullifying the tendency of the tap-point terminal to move. In other words, both the rate of rise of V_(PP) and the final value of V_(PP) can be controlled by the parameters of capacitors 22 and 24, current source 30, and the value of V_(ref), as described below in more detail.

40. In operation, charge pump system 2 is kept in a standby position until the high voltage V_(PP) is needed. In standby position, switch 6 connects voltage source V_(DD) to V_(DD)/V_(PP) bus 4 (so that voltage value V_(DD) appears on the bus), and switch 8 connects node A to ground (GND). Furthermore, switch 26 connects the tap-point terminal of voltage divider circuit 20 to node C, and switch 28 connects the bottom terminal of voltage divider circuit 20 also to node C. Capacitor 24 has no charge because both of its terminals are at reference voltage V_(ref).

41. Charge pump system 2 does not generate the high voltage V_(PP) until such voltage is needed. For example, if charge pump system 2 is incorporated into an EEPROM, high voltage V_(PP) would be needed for programming.

42. When the high voltage V_(PP) is required, charge pump system 2 is activated, and switch 6 is moved to disconnect voltage source V_(DD) from V_(DD)/V_(PP) bus 4, and switch 8 is moved to connect node A to the bus. Switch 26 is moved to connect the tap-point terminal of voltage divider circuit 20 to node B, and switch 28 is moved to connect the bottom terminal of circuit 20 to current source 30.

43. Both the tap-point terminal and the bottom terminal of voltage divider circuit 20, initially held in standby position at the fixed reference-voltage potential V_(ref), are released. Current source 30 generates I_(pulldown) which proceeds to pull the voltage at the bottom terminal toward ground potential. As the voltage at the bottom terminal is pulled down, the voltage at the tap point terminal attempts to follow, thus unbalancing comparator circuit 16 which compares the voltage at the tap-point terminal with the fixed reference potential V_(ref). Comparator circuit 16 outputs current I_(CNTL), which is proportional in magnitude to the difference between the voltages of the tap-point terminal and V_(ref) and decreases to zero at balance. I_(CNTL) establishes the frequency of oscillator circuit 14 which begins to drive charge pumping circuit 12. In response, charge pumping circuit 12 starts to raise V_(PP). That is, charge pumping circuit 12 begins to pump up the voltage at node A.

44. As charge pumping circuit 12 starts, the top terminal of voltage divider circuit 20 (connected to the output of the charge pumping circuit 12) rises from ground potential. As I_(pulldown) proceeds to pull down the bottom terminal of voltage divider circuit 20, the voltage at the tap point terminal attempts to follow. The change in V_(PP), coupling through capacitor 22 of voltage divider circuit 20, tends to cancel the movement of the tap point terminal. The feedback loop (supported by voltage divider circuit 20 and current source 30) causes the V_(PP) ramp rate to proportionally follow the rate of descent of the bottom terminal. When the bottom terminal reaches ground, V_(PP) stops rising. Since the change in charge of the top capacitor 22 must equal the change in the bottom capacitor 24, the V_(PP) level is set by the value (C1/C2)*V_(ref). The V_(PP) rise time, which is equal to the time for the bottom terminal to reach ground, is thus set by the value C2*V_(ref)/I_(pulldown). Accordingly, charge pump system 2 can be designed with suitable values for C2, V_(ref), and I_(pulldown) so that V_(PP) rises at a desired ramp rate.

45. Because of the feedback configuration, oscillator circuit 14 is controlled to run at the minimum frequency for clock signals Φ₁ and Φ₂ needed to sustain the ramp (typically less than 100 kHz), thus minimizing dynamic current drain. When V_(PP) stabilizes, oscillator circuit 14 stops completely, then briefly turns on only occasionally to accommodate any leakage current drawn from V_(PP).

46. To achieve the desired ramp rates, the required ratio of C2 to I_(pulldown) may be quite high. That is, because practical minimum values for controllable currents in IC devices lie in the range of 10-100 nA, capacitor 24 ordinarily would be required to be quite large (e.g., 200-300 pF). However, in accordance with an embodiment of the present invention, current source 30 may be controlled with duty-cycle so that the size of capacitor 24 may be reduced. Specifically, by using a periodic control with duty cycle (D) for I_(pulldown), the value of C2 can be decreased by a factor of D.

47. Charge Pumping Circuit

48.FIG. 2 is a schematic diagram of an exemplary embodiment for charge pumping circuit 12 shown in FIG. 1. As shown, charge pumping circuit 12 comprises a series of n-type transistors 32 coupled between V_(DD) and V_(PP). The gate of each of transistors 32 is coupled to the drain of the same transistor 32 and also to the gate of a separate n-type transistor 34. The source and drain of each transistor 34 is coupled to oscillator circuit 14 and receives either the Φ₁ or Φ₂ clock signal. With this arrangement, charge pumping circuit 12 comprises a series of rectifying devices which pump up the voltages of the nodes between V_(DD) and V_(PP) along the series comprising transistors 32.

49. Oscillator Circuit

50.FIG. 3 is a schematic diagram of an exemplary embodiment for oscillator circuit 14, in accordance with an embodiment of the present invention. Oscillator circuit 14 generates two non-overlapping, oscillating clock signals Φ₁ and Φ₂.

51. As depicted, oscillator circuit 14 comprises two current mirrors 36. Current mirrors 36 reflect the current I_(CNTL) which is output by comparator circuit 16 (FIG. 1) and directly proportional to the difference between V_(ref) and the scaled-down V_(PP) voltage. Each current mirror 36 comprises a p-type transistor 38 which receives the I_(CNTL) mirror voltage at its gate. Each current mirror 36 outputs a respective output current signal.

52. A separate inverter 40 is coupled a respective current mirror 36. Each inverter 40 comprises a p-type transistor 42 and an n-type transistor 44, the gates of which are coupled together at a respective node F. The output of each inverter 40 is coupled to a respective capacitor 46 at a respective node G. The values for capacitors 46, in conjunction with the values of the reflected I_(CNTL) currents, can be selected to provide a desired timing for the Φ₁ and Φ₂ clock signals.

53. A separate NOR gate 48 receives the output current signal from a respective current mirror 36 through a respective inverter 40; with this arrangement, each current mirror 36 controls the amount of current delivered to the respective capacitor 46, thus defining the voltage at the input of NOR gate 48. The gates of transistors 42 and 44 in each inverter 40 is coupled to the respective NOR gate 48 at the respective node F. The output of each NOR gate 48 appears at the respective node F of the other NOR gate 48. The output of one of NOR gates 48 is the Φ₁ clock signal, and output of the other NOR gate 48 is the Φ₂ clock signal.

54. Each NOR gate 48 comprises a first n-type transistor 50 and a second n-type transistor 52. If the voltage appearing at the gate of either of these transistors 50 or 52 is high, the value of the respective output clock signal is pulled low. For each NOR gate 48, a series of p-type transistors 54, 56, and 58 pull up the voltage value of the respective output clock signal.

55. With transistors 54, 56, and 58 connected as shown, hysteresis is created at nodes F. With this hysteresis, the trip point for each inverter 40 will differ depending on whether the voltage at the respective node F is increasing or decreasing. As such, each inverter 40 functions as a Schmitt trigger in which the output voltage level is binary, is determined by the magnitude of the input signal, and is independent of the input signal waveform. This provides stability against noise.

56. The design for oscillator circuit 14 described herein minimizes the number of moving nodes in charge pump system 2, thereby conserving dynamic current (the value of which is given by the equation I_(dynamic)=CVf as described above). Specifically, functioning as Schmitt triggers, inverters 40 monitor the voltages developed across timing capacitors 46, which are alternately charged by a current proportional to I_(CNTL) (provided by comparator circuit 16 shown in FIG. 1). Inverters 40 control the charging sequence, discharging one capacitor 46 while simultaneously monitoring the rising voltage on the other. When the rising voltage reaches the trip point for the Schmitt trigger, such inverter 40 switches and the two charging circuits change roles. The timing capacitors 46 can be made very small so that the current reflected from the frequency-control current I_(CNTL) from comparator circuit 16 can be minimized.

57. In this embodiment, oscillator circuit 14 carries out an oscillating operation by feeding back the outputs of inverters 40 with respective capacitors 46, thereby inverting the input signals to the same inverters. When the signals pass through the inverters 40, the timing is controlled so as to vary the oscillation frequency. That is, if the signal transmission time of an inverter 40 is extended, then the frequency of the respective clock signal (Φ₁ or Φ₂) is decreased, while if the signal transmission time is shortened, the frequency is increased.

58. Voltage Divider Circuit

59.FIG. 4 is a schematic diagram of circuit for an alternative voltage divider circuit 60, in accordance with another embodiment of the present invention. Voltage divider circuit 60 can be used as an alternative to voltage divider circuit 20 shown in FIG. 1. Voltage divider circuit 60 divides the voltage at node A (at which V_(PP) appears) , and can be used to generate a voltage level of approximately one-half V_(PP) (as described below in more detail).

60. As depicted, voltage divider circuit 60 comprises a plurality of capacitors 62, 64, and 66 coupled in series between node A and a node H. Capacitors 62, 64, and 66 have respective values of “C1a,” “C1b,” and “C2.” A first tap point terminal is located between capacitors 64 and 66, and a second tap point terminal is located between capacitors 62 and 64. At the first tap point terminal, voltage divider circuit 60 can be connected to comparator circuit 16 (FIG. 1).

61. A current source 68, coupled between capacitor 66 and ground, outputs current I_(pulldown) which pulls down the voltage at node H. An n-type transistor 70 is coupled at its drain to node A, at its gate to the second tap point terminal, and at its source to a node I. At node I, the voltage value of approximately one-half V_(PP) can be generated by voltage divider circuit 60. A switch 72 can be closed to connect node I to ground.

62. In operation, when charge pump system 2 is active, switch 72 is open so that node I “floats.” The voltage at the first tap point terminal is maintained at approximately the value of the reference voltage (V_(ref)). The voltage at the second tap point terminal is a divided or scaled-down replica of V_(PP). This voltage appears at the gate of transistor 70 which functions as a source follower. Transistor 70 translates the voltage at the second tap point to a voltage of approximately one-half V_(PP) at node I.

63. One purpose for generating a voltage of approximately one-half V_(PP) is as follows. Some semiconductor technologies employ p-type transistors incapable of withstanding drain-to-source voltages as high as V_(PP). In such case, two p-type transistors are used. The p-type transistors are connected in series as an upper transistor and a lower transistor. The upper transistor, with its source and its N-well substrate connected to V_(PP), is the active switching transistor. The lower transistor has a separate N-well substrate connected to its source. The gate of the lower transistor is connected to a non-switching bias voltage equal to approximately one-half V_(PP) voltage so that its source and substrate can rise no higher than a p-channel threshold above the half-V_(PP) voltage level. Accordingly, neither transistor is required to support a voltage of more than one-half V_(PP).

64. With many previously developed techniques, the half-V_(PP) bias voltage is derived from V_(PP) using a resistive divider. Such resistive divider may be implemented with linear resistors or stacked, diode-connected transistors. In either case, the resistive divider draws quasi-static current (I_(PP)) from V_(PP). Because charge-pump efficiency (I_(PP)/I_(DD)) inherently is low, current drain I_(DD) from supply voltage V_(DD) is significantly increased.

65. In contrast to previous techniques, voltage divider circuit 60 depicted in FIG. 4 provides a one-half V_(PP) voltage with no I_(DD) current demand. With the capacitor divider of capacitors 62, 64, and 66, the second tap point terminal can be used as the input to the source follower (comprising transistor 70) powered by V_(PP). The output of the source follower then provides a voltage of approximately one-half V_(PP). No pulldown is required for the source follower as V_(PP) rises, and hence it draws no current from V_(PP). Node I is pulled to ground when charge pump system 2 is not active. Thus, a voltage value of one-half V_(PP) is made available with no current drain on V_(PP).

66. Switch Circuit and Comparator Circuit

67.FIG. 5 is a schematic diagram of an exemplary embodiment for a switch circuit 74 and a comparator circuit 76, in accordance with an embodiment of the present invention. Switch circuit 74 and comparator circuit 76 can be used to implement the switching function of switches 6 and 8 of charge pump system 2.

68. Switch circuit 74 enables node A (at which V_(PP) appears) to be at ground potential and V_(DD)/V_(PP) bus 4 to be at V_(DD) potential during standby position (and at the beginning of active position). Switch circuit 74 comprises a first n-type transistor 76 and a second n-type transistor 78, the gates of which are coupled together at a node J. One end of each of transistors 76 and 78 is coupled to node A; the opposing end of transistor 78 is coupled directly to V_(DD)/V_(PP) bus 4, and the opposing end of transistor 76 is coupled through a resistor 80 to V_(DD)/V_(PP) bus 4.

69. Comparator circuit 76 compares the voltage value of V_(PP) at node A against the voltage value of supply V_(DD). Comparator circuit 76 comprises n-type transistors 82 and 84. V_(PP) is applied at the gate of transistor 82, and V_(DD) is applied at the gate of transistor 84. The drains of transistors 82 and 84 are connected to V_(DD). The sources of transistors 82 and 84 are connected to the sources of p-type transistors 86 and 88 at nodes K and L, respectively. In this arrangement, transistors 82 and 84 operate as source followers. That is, the voltage at node K follows V_(PP), and the voltage at node L follows V_(DD). The gates of transistors 86 and 88 are connected together, and coupled to the drain of transistor 88. As such, transistors 86 and 88 implement a current mirror. The drains of transistors 86 and 88 are connected to the drains of n-type transistors 92 and 94, respectively. The dimensions for transistors 92 and 94 can be selected so that the current which flows through transistor 86 is somewhat larger than (e.g., twice) that which flows through transistor 88. The sources of transistors 92 and 94 are connected to ground, and the gates of these transistors receive a current-mirror reference voltage. The gate of an n-type transistor 90 is connected to the gates of transistors 86 and 88. The source and drain of transistor 90 are connected to ground. Transistor 90 serves as a capacitor, helping to hold the voltage constant at the gate of transistor 86.

70. An inverter 96 is coupled at its input terminal (at a node M) to comparator circuit 76. The output terminal of inverter 96 is connected to node J so that the inverter output is applied to the gates of transistors 76 and 78 of switch circuit 74.

71. In operation, switch circuit 74 allows the voltage V_(PP) at node A to be at ground potential in standby (as required by charge pump system 2 of FIG. 1) while connecting V_(DD)/V_(PP) bus 4 to V_(DD) at that time. In particular, when the charge pump system 2 is turned off (i.e., switched from active position to standby position), a path is established to pull the pump output at node A to ground at a designed rate of decline. V_(DD)/V_(PP) bus 4 is connected to node A, and accordingly, is discharged. When the voltage potential of V_(DD)/V_(PP) bus 4 reaches V_(DD), switch circuit 74 opens the path between node A and V_(DD)/V_(PP) bus 4 and substitutes a path to V_(DD) via transistors 96 and 100.

72. In operation for comparator circuit 76, if V_(PP) is equal to V_(DD), which is approximately the trip point for comparator circuit 76, then the voltages at nodes K and L will be approximately equal. If transistors 86 and 88 are equal in size and the current which flows through transistor 86 is twice as large as the current which flows through transistor 88, the voltage at node M is pulled to ground. On the other hand, if V_(PP) is greater than V_(DD), the voltage at node K will be greater than the voltage at node L. Transistor 86 can conduct more current because its source is being held higher. This causes the voltage at node M to go to a high value. As described herein, comparator circuit 76 operates without drawing current from V_(DD)/V_(PP) bus 4.

73. The embodiments of the present invention described herein minimize or reduce current drain in a charge pump system which outputs a high voltage V_(PP). With these embodiments, a total supply current (I_(DD)) of less than 50 uA is required. Accordingly, the present invention provides a distinct technical advantage over previously developed techniques and systems.

74. While particular embodiments of the present invention have been shown and described, it will be obvious to those skilled in the art that changes and modifications may be made without departing from this invention in its broader aspects and, therefore, the appending claims are to encompass within their scope all such changes and modifications as fall within the true spirit and scope of this invention. 

What is claimed is:
 1. A charge pump system comprising: a charge pumping circuit operable to output a high voltage V_(PP) at a node; an oscillator circuit coupled to the charge pumping circuit, the oscillator circuit operable to drive the charge pumping circuit with at least one clock signal; a current source operable to generate a pulldown current; and a voltage divider circuit coupled between the node and the current source, the voltage divider circuit cooperating with the current source to form a feedback loop for controlling the oscillator circuit to run at variable, optimum frequency for controlling the rate-of-rise and the amplitude of the high voltage V_(PP).
 2. The charge pump system of claim 1 wherein the voltage divider circuit comprises a first capacitor and a second capacitor coupled in series between the node and the current source.
 3. The charge pump system of claim 1 wherein the voltage divider circuit divides the high voltage V_(PP) to produce a scaled-down voltage.
 4. The charge pump system of claim 3 further comprising a comparator circuit coupled to the oscillator circuit and the voltage divider circuit, the comparator circuit operable to compare the scaled-down voltage against a reference voltage and to output a control current in response.
 5. The charge pump system of claim 1 wherein the oscillator circuit comprises: a current mirror operable to generate an output current signal; and a NOR gate receiving the output current signal from the current mirror, the NOR gate operable to output the at least one clock signal.
 6. The charge pump system of claim 5 wherein the oscillator circuit further comprises an inverter coupled between the current mirror and the NOR gate.
 7. An oscillator circuit comprising: a current mirror operable to receive a control signal and to generate an output current signal in response; an inverter coupled to the current mirror; a capacitor receiving the output current signal through the inverter; and a NOR gate receiving the voltage developed across the capacitor and generating at least one clock signal.
 8. The oscillator circuit of claim 7 wherein the inverter is operable to control charging and discharging of the capacitor.
 9. A voltage divider circuit comprising: a first capacitor coupled to a first node at which a high voltage V_(PP) appears; a second capacitor coupled in series to the first capacitor at a second node; a third capacitor coupled in series to the second capacitor at a third node; and a source follower coupled to the second node, the source follower operable to translate a voltage at the second node to a voltage having a value of approximately one-half V_(PP).
 10. The voltage divider circuit of claim 9 wherein the source follower comprises a transistor having a gate coupled to the second node, one end coupled to the first node, and another end coupled to a fourth node at which the voltage having a value of approximately one-half V_(PP) appears.
 11. The voltage divider circuit of claim 9 further comprising a current source coupled to the third capacitor at a fifth node, the current source operable to generate a pulldown current for pulling down a voltage at the fifth node.
 12. A comparator circuit comprising: a first source follower which follows a supply voltage V_(DD); a second source follower which follows a high voltage V_(PP); and a current mirror coupled to the first and second source followers, the current mirror for pulling the voltage at a node to a first value if the high voltage V_(PP) is approximately equal to the supply voltage V_(DD), the current mirror for pulling the voltage at the node to a second value if the high voltage V_(PP) is greater than the supply voltage V_(DD), thereby comparing the high voltage V_(PP) to the supply voltage V_(DD) without drawing current from the high voltage V_(PP).
 13. The comparator circuit of claim 12 wherein each of the first and second source followers comprises an n-type transistor.
 14. The comparator circuit of claim 12 wherein the current mirror comprises two p-type transistors.
 15. The comparator circuit of claim 12 further comprising: a first transistor coupled to one branch of the current mirror; a second transistor coupled to another branch of the current mirror; and the first and second transistors having dimensions such that current flowing through the first transistor is larger than that flowing through the second transistor. 